In the combustion of a fuel, such as coal, oil, natural gas, peat, waste, etc., in a combustion plant, such as a power plant or a waste incineration plant, process gases are generated. For separating nitrogen oxides, usually denoted NOx, from such process gases, often referred to as flue gases, a method is frequently used, in which a reducing agent, usually ammonia or urea, is mixed with the flue gases. The flue gases, mixed with said ammonia or urea, are then passed through a catalyst in which the ammonia reacts selectively with the NOx to form nitrogen gas and water vapour. Usually the catalyst is installed in a so called Selective Catalytic Reduction reactor (SCR-reactor).
In many processes, in particular waste incineration plants and peak load power plants, the load of NOx that needs to be handled by the SCR-reactor varies heavily over time. The variations in NOx load depend, in particular, on varying fuel qualities and varying supply, i.e., varying load, of fuel to the waste incineration plant or the power plant.
EP 0 604 236 B1 describes a power plant utilizing a gas turbine. It is recognized, in EP 0 604 236 B1, that the standard technique of controlling the dosage of ammonia to the flue gases, upstream of the SCR-reactor, is to measure the concentration of NOx downstream of the SCR-reactor and to utilize a feed-back control for controlling the dosage of ammonia to the flue gases. EP 0 604 236 B1 notes that such a system has a very slow response time to load changes. EP 0 604 236 B1 proposes to utilize a feed-forward controller which controls the supply of ammonia based on a measurement of the concentration of NOx upstream of the SCR-reactor.
Even with the feed-forward control of EP 0 604 236 B1 the response to load changes is sometimes not quick enough, which can result in an emission, to the ambient air, of NOx or ammonia which is in an amount that is above the emission limit, which has been set by an environmental authority.